Swimming of a ludion in a stratified sea*

We describe and model experimental results on the dynamics of a "ludion" - a neutrally buoyant body - immersed in a layer of stably stratified water. The ludion or Cartesian diver, initially positioned at its equilibrium height and free to move horizontally, can oscillate vertically when forced by pressure oscillations caused by the motion of a piston. Depending on the ratio of the forcing frequency to the buoyancy frequency, the ludion can emit its own internal gravity waves that we measure by PIV. Our experimental results describe first the resonance of the vertical motions of the ludion when excited at different frequencies. A theoretical oscillator model is then derived taking into account added mass and added friction coefficients and its predictions are compared to the experimental data. Then, for the larger oscillation amplitudes, we observe a bifurcation towards free horizontal motions. Although the internal gravity wave frequencies are affected by the Doppler shift induced by the horizontal displacement velocities, they are not the cause of the horizontal swimming. This does not however, exclude possible interactions between the ludion and his internal gravity waves and possible hydrodynamic quantum analogies as already observed with the bouncing drops of Couder.